Guiomar is the best known name of a character appearing in many medieval texts relating to the Arthurian legend, often in relationship with Morgan le Fay or a similar fairy queen type character.

Morgan le Fay, alternatively known as Morgan[n]a, Morgain[a/e], Morg[a]ne, Morgant[e], Morge[i]n, and Morgue[in] among other names and spellings, is a powerful and ambiguous enchantress from the legend of King Arthur, in which most often she and he are siblings. Early appearances of Morgan in Arthurian literature do not elaborate her character beyond her role as a goddess, a fay, a witch, or a sorceress, generally benevolent and connected to Arthur as his magical saviour and protector. Her prominence increased as the legend of Arthur developed over time, as did her moral ambivalence, and in some texts there is an evolutionary transformation of her to an antagonist, particularly as portrayed in cyclical prose such as the Lancelot-Grail and the Post-Vulgate Cycle. A significant aspect in many of Morgan's medieval and later iterations is the unpredictable duality of her nature, with potential for both good and evil.